MWS-BROAD selection needs a colour cut

[apcooper]

@geordie666, a minor correction to the changes in respect of #410. Not urgent, can be fixed in the next round of updates.

The MWS-BROAD targets should also be limited to g-r > 0.7, like the MAIN-RED targets. We want BROAD to be the complement of the astrometric cuts that only apply to RED. We don't apply any of the astrometry cuts to BLUE in the first place, so we don't want all the blue targets that fail those cuts to be counted as BROAD as well as BLUE.

However, it looks like the code added in #418 doesn't include that colour cut for BROAD:

desitarget/py/desitarget/cuts.py

Lines 686 to 693 in eb1c2d2

	broad = mws.copy()
	# ADM MWS-RED also has parallax < 1mas and proper motion < 7.
	red &= pm < 7.
	red &= parallax < 1.
	# ADM MWS-BROAD has parallax > 1mas OR proper motion > 7.
	broad &= (parallax >= 1.) | (pm >= 7.)

Rather thanbroad = mws.copy() I think this should have broad = red.copy() in the same place (i.e. before the astrometry cuts are applied to red).

This is what we wrote on the TS page, but sorry I didn't catch it in the PR.